Aircraft braking systems are generally designed to operate in several modes:                a normal mode during which a braking control unit generates a braking setpoint for power delivery members (pressure servo-valve for hydraulic braking, a controller for electromechanical braking) that deliver a determined amount of power to the braking actuators;        an alternative mode in which the primary power circuit has failed, and power is taken from a second power circuit;        an emergency mode in which the braking control unit has failed, with the braking order then being transmitted directly from the pedals to the power distribution members, and thus without any anti-skid protection; in general, the power available at the brakes is deliberately limited in order to prevent the wheels from locking; and        finally, in the last resort, an ultimate braking mode that consists in using parking braking for slowing down the aircraft.        
It should be recalled that in hydraulically braked aircraft, parking braking consists in putting the cavities of the brakes into communication with an accumulator, with the pressure in that accumulator sufficing to deliver enough parking braking force to prevent the aircraft from moving. Parking braking (and thus ultimate braking) has priority over the other modes of braking, and in practice this priority is implemented very simply by means of a shuttle valve arranged upstream from the brakes to put them into communication either with the outlet from the parking accumulator, or with the outlet from the pressure servo-valve controlled by the braking control unit. In order to be certain that priority is complied with, the pressure in the parking accumulator is generally high, such that it forces the shuttle valve to switch, even if the pressure servo-valve attempts to impose its own pressure. It should be observed that upstream from the shuttle valve, the two control systems (using the braking control unit or the parking selector) are completely separate, in order to ensure the greatest possible segregation.
In aircraft having electric brakes, the situation is not that simple. Parking braking is obtained by causing the electromechanical braking actuators to apply a force, and then by causing the actuators to be blocked in position.
As with hydraulic braking systems, it is tempting to use parking braking as means for ultimate braking enabling the aircraft to be slowed down when all other braking means have become unavailable. Nevertheless, if for any reason the event that has lead to parking braking being used as ultimate braking should disappear (e.g. successfully rebooting the braking control unit so that it becomes operational once more), the blocking of the actuators can be problematic. For segregation reasons, the blocking logic is generally not controlled by the braking control unit, so the braking control unit has no way of unblocking the actuators. If the pilot does not remember to move the parking selector in order to cancel the parking order, then the actuators remain blocked.